This invention relates generally to wells such as oil and gas wells and especially to chemical treatment of such wells and the subterranean formations surrounding such wells. More particularly, the invention relates to the type of chemical treatment known in the art as "acidization."
Acidization of wells is a well known process for increasing or restoring the permeability of subterranean formations and thereby facilitating the flow of well fluids such as oil or gas into the well, and also facilitating the injection of fluids into the formation from the well if such is desired.
Acidization, as the term implies, generally consists of treating the formation with an acid, usually hydrochloric acid. The acid is effective to dissolve certain clogging deposits such as carbonate to open pores and other flow channels thereby increasing, sometimes remarkably, the permeability of the formation.
Many different acidizing procedures have been suggested in the prior art to cope with many different well conditions and special formation or well problems. But one condition which has continued to present a special problem is the extreme high temperatures present in many formations, especially in connection with very deep wells. In recent years more and more very deep high temperature wells have been drilled. Acid is much more reactive at increased temperatures. Generally, acid reactivity doubles with an increase of eighteen fahrenheit degrees. Thus, at 300.degree.F the reactivity of an acid may be about 4000 times what it is at 80.degree.F.
In any acidization process, a problem exists in protecting well equipment such as tubing and casing from corrosion by the acid. For this purpose, corrosion inhibitors are generally added to the acid solution prior to injection of the solution into the borehole. But it is difficut to find inhibitors which are effective at very high temperatures. And the expense of the inhibitor additives, which may be significant even for the types and amounts required at low temperatures, may be prohibitive at very high temperatures.
Another difficulty with inhibitors, especially in the quantities necessary in high temperature wells, is their tendency to form insoluble solids upon reaction with certain formation materials, thereby causing damage to the formation.
This invention provides an effective acidization method which avoids the above-mentioned problems and yet provides significant increases in permeability. The methods provided are especially useful in high temperature wells but may also be useful in other contexts of use.